1. Field of the Invention
The present invention relates to bituminous roofing products such as asphalt shingles and processes for making such roofing products.
2. Brief Description of the Prior Art
Roofing products such as asphalt shingles are typically composite articles including a nonwoven glass fiber or felt web covered with a coating of water repellent bituminous material, and surfaced with protective mineral-based roofing granules.
The bituminous material is characteristically black in color, and is strongly absorptive of incident solar radiation.
Pigment-coated mineral rocks are commonly used as color granules in roofing applications to provide aesthetic as well as protective functions to the asphalt shingles. Roofing granules are generally used in asphalt shingle or in roofing membranes to protect asphalt from harmful ultraviolet radiation.
Mineral surfaced asphalt shingles, such as those described in ASTM D225 or D3462, are generally used in steep-sloped roofs to provide water-shedding function while adding aesthetically pleasing appearance to the roofs. The asphalt shingles are generally constructed from asphalt-saturated roofing felts and surfaced by pigmented color granules, e.g., as those described in U.S. Pat. No. 4,717,614. However, such asphalt shingles are known to have low solar reflectivity and hence will absorb solar heat especially through the near infrared range of the solar spectrum.
This phenomenon increases as the granules covering the surface become dark in color. For example, the white-colored asphalt shingles with CIE L*>60 can have solar reflectance greater than 25% (ASTM E1918 method), whereas the non-white asphalt shingles with L*<60 can only have solar reflectance in the range of 5-20%. As a result, it is common to measure temperatures as high as 160-170° F. (71-77 degrees Centigrade) on the surface of dark roofing shingles in a sunny day with 80° F. (27 degree Centigrade) ambient temperature.
Absorption of solar heat may result in elevated temperatures at the shingles' surroundings, which can contribute to the so-called “heat-island” effects and increase the energy load required to cool the surroundings.
In order to address this problem, externally applied coatings have sometimes been applied directly onto the shingle surface on the roof. White pigment-containing latex coatings have been proposed.
The use of exterior-grade coatings colored by infrared-reflective pigments for deep-tone colors has also been proposed for spraying onto the roof in the field. U.S. Patent Application Publication No. 2003/0068469A1 discloses an asphalt-based roofing material comprising mat saturated with asphalt coating and a top coating having a top surface layer that has a solar reflectance of at least 70%.
U.S. Patent Application Publication No. 2002/0160151A1 discloses an integrated granule product comprising a film having a plurality of ceramic-coated granules bonded to the film by a cured adhesive and the cured adhesive or the film can have pigments. Such integrated granule product can be directly bonded to an asphalt-based substrate as roofing products.
Roofing granules typically comprise crushed and screened mineral materials, which are subsequently coated with a binder containing one or more coloring pigments, such as suitable metal oxides. The binder can be a soluble alkaline silicate that is subsequently insolubilized by heat or by chemical reaction, such as by reaction between an acidic material and the alkaline silicate, resulting in an insoluble colored coating on the mineral particles. Preparation of colored, coated roofing granules is disclosed for example, in U.S. Pat. No. 2,981,636 of Lodge et al. The granules are then employed to provide a protective layer on asphaltic roofing materials such as shingles, and to add aesthetic values to a roof.
Pigments for roofing granules have usually been selected to provide shingles having an attractive appearance, with little thought to the thermal stresses encountered on shingled roofs. However, depending on location and climate, shingled roofs can experience very challenging environmental conditions, which tend to reduce the effective service life of such roofs. The elevated temperature experienced by roofing shingles under sunny, summer conditions is a significant environmental stress, especially for roofing shingles coated with dark colored roofing granules. Although such roofs can be coated with solar reflective paint or coating material, such as a composition containing a significant amount of titanium dioxide pigment, in order to reduce such thermal stresses, this utilitarian approach will often prove to be aesthetically undesirable, especially for residential roofs.
Asphalt shingles coated with conventional roofing granules are known to have low solar heat reflectance, and hence will absorb solar heat especially through the near infrared range (700 nm-2500 nm) of the solar spectrum. This phenomenon increases as the granules covering the surface become dark in-color.
For example, while white-colored asphalt shingles can have solar reflectance in the range of 25-35%, dark-colored asphalt shingles can have solar reflectance in the range of only 5-15%. Furthermore, except in the white or very light colors, there is typically only a very small amount of pigment in the conventional granule's color coating that reflects solar radiation well.
There is a need for an asphalt shingle that has solar reflectivity greater than 25% to reduce the solar heat absorption, while providing aesthetically pleasing, non-white colors to maintain the aesthetic value of roofing assembly.
There is a continuing need for roofing materials, and especially asphalt shingles, that have improved resistance to thermal stresses while providing an attractive appearance.
In particular, there is a need for roofing shingles that provide increased solar heat reflectance to reduce the solar absorption of the shingles, while providing a wide range of colors including deep-tone colors to maintain the aesthetic value of the system.